if you ask Rufus (the guy who manages the MCG's patreon page), he'll say that it's down to "availability" and "popularity", that models like Danielle and Poppy are more available to shoot than other models (Stacey, Alice, etc.). He even admitted once that when a model cancels a shoot, they always turn to Danielle and Poppy to fill the void. Also, Danielle and Poppy are more popular than Stacey or Alice (or even Serenity?). I find that hard to believe.
Though, even if that were true, that still doesn't explain the endless amount of content they have of both of her. Personally, I still think they're just "in love" with both of them, and this "love" makes them overlook other models. They had Jen Loveheart for a bit and completely messed that up. I was hoping to see more of Ruby Mae, but they also seem to mess this one up. Even the latest additions to the roster has been underwhelming.
They defo need to shake thing up. Add more verity of models, stop spamming the same two models, and let the girls be more themselves and have more fun in the shoots. Those blank stares and dead-serious expressions aren't sexy.